As a conventional method of taking a photograph of a proper brightness, there is known a method of measuring the average luminance of a photographing scene and controlling the shutter speed and aperture value of a camera or the like. There is also known an exposure control method using so-called multiple-zone evaluative light-metering in which a scene is divided into predetermined regions and luminance levels measured for the respective regions are weighted to calculate an average luminance, thereby obtaining correct exposure.
However, a main object in a photographed image becomes dark in a so-called backlight scene in which the brightness of the main object to be photographed is much lower than that of the background. To take a photograph of a proper background in such a backlight scene, exposure correction must be done to set the exposure of the camera for a higher brightness than that for an average photograph in photographing. However, this exposure correction operation is cumbersome and needs a lot of skill in order to appropriately set the camera. Even if exposure correction is properly performed for a main object, the background becomes too bright.
As a method which solves this problem, analog photography adopts dodging in a darkroom to attain a print of a proper brightness. To easily implement dodging, it is desirably done by digital image processing.
For this purpose, the present applicant has proposed a method in Japanese Patent Application No. 2003-199125 in which the degree of luminance level enhancement is given as a parameter and the brightness distribution is corrected. This technique can enhance the brightness of the dark region of an image, but there is a room for improvement in the tone reproducibility of the bright region.
As a method which realizes this process, Japanese Patent Application Laid-Open No. 10-13680 discloses an image processing method of independently nonlinearly compressing bright and dark regions without changing a halftone region in order to reproduce the tones of both the bright and dark regions.
However, the user often wants to keep brightnesses of different regions unchanged depending on the image. For example, when a person is photographed in a backlight scene with a bright background, it is desirable to keep the brightness of a region slightly darker than a relatively bright sky region as it is. In a scene with a dark background such as a night scene, it is desirable to keep the brightness of a very dark region intact. The technique disclosed in Japanese Patent Laid-open No. 10-13680, however, cannot control (change) a region whose brightness is to be maintained, and cannot cope with various photographing scenes.